Most conventional light wavelength measuring apparatuses employ a Michelson interferometer. The basic constitution of a light wavelength measuring apparatus using the Michelson interferometer is shown in FIG. 1
In FIG. 1, the measured an incoming light I/P (wavelength: .lambda.in) is mixed with reference light R/P (wavelength: .lambda.ref) whose wavelength is considerably shorter than that of the measured light I/P by a first half mirror 81, and the mixed light is distributed to two systems by a second half mirror 82 of the Michelson interferometer.
The light of one of the systems is reflected by a fixed mirror 83 and then returned to the second half mirror 82. The light of the other system is incident upon a movable mirror 84. The movable mirror 84 periodically moves back and forth along the optical axis within a predetermined range. As a result, the light incident upon the movable mirror 83 is returned to the second half mirror 82 owing to the movement of the movable mirror.
The reflected light of the fixed mirror 83 and that of the movable mirror 84 are mixed with each other by the second half mirror 82, and the mixed light enters a WDM (Wavelength-Division Multiplexing) device 85 and is divided for each wavelength. The light of wavelength .lambda.in and that of wavelength .lambda.ref are converted into electrical signals by photoelectric converters (photodiodes) 86 and 87, respectively, and these signals are transmitted to counter circuits 88 and 89.
The counter circuits 88 and 89 detect the respective peak timings of the input signals to count peak timing signals of the light components during one cycle of movement of the movable mirror 84. The values counted by the counter circuits 88 and 89 are supplied to a wavelength calculation unit 810.
The wavelength calculation unit 810 calculates the number of peaks of wavelength interference patterns of the measured light and reference light on the basis of a ratio of the values of measured light components counted by the counter circuit 88 to those of reference light components counted by the counter circuit 89, thereby measuring the wavelength of the measured light based on the number of peaks.
Since, however, the above-described light wavelength measuring apparatus using the Michelson interferometer has a mechanical movable section for moving the mirror, the maximum measurement speed is restricted to about 10 times per second, and the movable section greatly influences the reliability of the apparatus. The maximum measurement speed and the reliability are particularly difficult conditions for optical communication systems requiring MTBF (Mean Time Between Failures) of several years.
Furthermore, since, in the foregoing light wavelength measuring apparatus, the wavelength of measured light is measured based on the number of peaks of wavelength interference patterns of the measured light and reference light, the wavelength of the reference light has to be stable and precise. This is also a very difficult condition, and the apparatus is forced to use a large-sized, expensive He-Ne (helium-neon) laser as a reference light source to achieve high wavelength stability. The apparatus is therefore increased in size and cost.
As described above, since the conventional light wavelength measuring apparatus has the mechanical movable section, the maximum measurement speed is restricted to remarkably low values, and the reliability of the apparatus deteriorates. Since, furthermore, the stability of wavelength is required for the reference light source, a large-sized, expensive laser has to be used, thereby increasing the size and cost of the whole apparatus.